(119h) Effects of Elasticity and Flow Ramp up on Kinetics of Shear Banding Flow Formation in Wormlike Micellar Fluids

When a steady shear is applied to a shear banding wormlike micellar solution, the solution can undergo a back flow, where some fluid moves opposite to the direction of the applied shear before the steady-state shear banding is established. In this work, we studied systematically the effects of varying two dimensionless parameters on the presence or absence of such back flows: first, elasticity number, which compares inertial to elastic forces, was varied by varying salt/surfactant concentrations and temperature, and second, initial flow ramp up rate was varied by specifying a ramp in the shear rate profile. Entanglement ratio and zero-shear viscosity were held fixed at either 6.7 ± 0.7 and 45.0 ± 7.8 Pa∙s, respectively, or 16.0±0.8 and 305±52 Pa∙s. For the set with lower entanglement ratio, fluid undergoes back flow only for elasticity number greater than 10⁶, while at 8.5 × 10⁶, transient flow evolution is more complex, and the back flow disappears. For the set with higher entanglement density, back flows are observed for elasticity numbers of 3.5 × 10⁶ and greater, with the greatest measured being 2.1 × 10⁸. Additionally, back flows are only observed for ramp up rates beyond a critical value, which depends on the elasticity number. Contrasting results for the two sets with different entanglement densities suggests that entanglement density significantly affect the transient evolution of the shear banding flows. Finally, under some conditions, steady state velocity profiles with three or more shear bands are observed.